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SQL概述</h2> <h3 id="_1-1-sql背景知识"><a href="#_1-1-sql背景知识" class="header-anchor">#</a> 1.1 SQL背景知识</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049310.png" alt="image-20220124003559327"></p> <h3 id="_1-2-sql语言排行榜"><a href="#_1-2-sql语言排行榜" class="header-anchor">#</a> 1.2 SQL语言排行榜</h3> <p>自从 SQL 加入了 TIOBE 编程语言排行榜，就一直保持在 Top 10。</p> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049313.png" alt="image-20220124003655718"></p> <h3 id="_1-3-sql-分类"><a href="#_1-3-sql-分类" class="header-anchor">#</a> 1.3 SQL 分类</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049315.png" alt="image-20220124003746154"></p> <h2 id="_2-sql语言的规则与规范"><a href="#_2-sql语言的规则与规范" class="header-anchor">#</a> 2. SQL语言的规则与规范</h2> <h3 id="_2-1-基本规则"><a href="#_2-1-基本规则" class="header-anchor">#</a> 2.1 基本规则</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049316.png" alt="image-20220609025050298"></p> <h3 id="_2-2-sql大小写规范-建议遵守"><a href="#_2-2-sql大小写规范-建议遵守" class="header-anchor">#</a> 2.2 SQL大小写规范 （建议遵守）</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049317.png" alt="image-20220609025112116"></p> <h3 id="_2-3-注-释"><a href="#_2-3-注-释" class="header-anchor">#</a> 2.3 注 释</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049318.png" alt="image-20220609025134235"></p> <h3 id="_2-4-命名规则-暂时了解"><a href="#_2-4-命名规则-暂时了解" class="header-anchor">#</a> 2.4 命名规则（暂时了解）</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049319.png" alt="image-20220609025241046"></p> <h3 id="_2-5-数据导入指令"><a href="#_2-5-数据导入指令" class="header-anchor">#</a> 2.5 数据导入指令</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049320.png" alt="image-20220609025330580"></p> <h2 id="_3-基本的select语句"><a href="#_3-基本的select语句" class="header-anchor">#</a> 3. 基本的SELECT语句</h2> <h3 id="_3-0-select"><a href="#_3-0-select" class="header-anchor">#</a> 3.0 SELECT...</h3> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">#没有任何子句</span>
<span class="token keyword">SELECT</span> <span class="token number">9</span><span class="token operator">/</span><span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">#没有任何子句</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_3-1-select-from"><a href="#_3-1-select-from" class="header-anchor">#</a> 3.1 SELECT ... FROM</h3> <ul><li>语法：</li></ul> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span>  标识选择哪些列
<span class="token keyword">FROM</span>   标识从哪个表中选择
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><ul><li>选择全部列：</li></ul> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token operator">*</span>
<span class="token keyword">FROM</span>  departments<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049321.png" alt="image-20220124015600153"></p> <blockquote><p>一般情况下，除非需要使用表中所有的字段数据，最好不要使用通配符‘*’。使用通配符虽然可以节省输入查询语句的时间，但是获取不需要的列数据通常会降低查询和所使用的应用程序的效率。通配符的优势是，当不知道所需要的列的名称时，可以通过它获取它们。
<strong>在生产环境下，不推荐你直接使用 SELECT * 进行查询。</strong></p></blockquote> <ul><li>选择特定的列：</li></ul> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> department_id<span class="token punctuation">,</span> location_id
<span class="token keyword">FROM</span>  departments<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049323.png" alt="image-20220124015657771"></p> <blockquote><p>MySQL中的SQL语句是不区分大小写的，因此SELECT和select的作用是相同的，但是，许多开发人员习惯将关键字大写、数据列和表名小写，读者也应该养成一个良好的编程习惯，这样写出来的代码更容易阅读和维护。</p></blockquote> <h3 id="_3-2-列的别名"><a href="#_3-2-列的别名" class="header-anchor">#</a> 3.2 列的别名</h3> <ul><li><p>重命名一个列</p></li> <li><p>便于计算</p></li> <li><p>紧跟列名，也可以在列名和别名之间加入关键字AS，<code>别名使用双引号</code>，以便在别名中包含空格或特殊的字符并区分大小写。</p></li> <li><p>AS 可以省略</p></li> <li><p>建议别名简短，见名知意</p></li> <li><p>举例</p> <blockquote><p>AS 全称alias(别名) <strong>有歧义的类名使用双引号<code>&quot;&quot;(规范)</code></strong></p></blockquote> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> last_name <span class="token keyword">AS</span> name<span class="token punctuation">,</span> commission_pct comm
<span class="token keyword">FROM</span>  employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049324.png" alt="image-20220124015828808"></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> last_name <span class="token string">&quot;Name&quot;</span><span class="token punctuation">,</span> salary<span class="token operator">*</span><span class="token number">12</span> <span class="token string">&quot;Annual Salary&quot;</span>
<span class="token keyword">FROM</span>  employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049325.png" alt="image-20220124015851699"></p></li></ul> <h3 id="_3-3-去除重复行"><a href="#_3-3-去除重复行" class="header-anchor">#</a> 3.3 去除重复行</h3> <p>默认情况下，查询会返回全部行，包括重复行。</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> department_id
<span class="token keyword">FROM</span>  employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049326.png" alt="image-20220124020013299"></p> <p><strong>在SELECT语句中使用关键字DISTINCT去除重复行</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token keyword">DISTINCT</span> department_id
<span class="token keyword">FROM</span>  employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049327.png" alt="image-20220124020037250"></p> <p>针对于：</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token keyword">DISTINCT</span> department_id<span class="token punctuation">,</span>salary
<span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这里有两点需要注意：</p> <blockquote><ol><li><code>DISTINCT 需要放到所有列名的前面</code>，如果写成 <code>SELECT salary, DISTINCT department_id FROM employees</code> 会报错。</li> <li>DISTINCT 其实是对后面所有列名的组合进行去重，你能看到最后的结果是 74 条，因为这 74 个部门id不同，都有 salary 这个属性值。如果你想要看都有哪些不同的部门（department_id），只需要写 <code>DISTINCT department_id</code> 即可，后面不需要再加其他的列名了。</li></ol></blockquote> <h3 id="_3-4-空值参与运算"><a href="#_3-4-空值参与运算" class="header-anchor">#</a> 3.4 空值参与运算</h3> <ul><li>所有运算符或列值遇到null值，运算的结果都为null</li></ul> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> employee_id<span class="token punctuation">,</span>salary<span class="token punctuation">,</span>commission_pct<span class="token punctuation">,</span> <span class="token number">12</span> <span class="token operator">*</span> salary <span class="token operator">*</span> <span class="token punctuation">(</span><span class="token number">1</span> <span class="token operator">+</span> commission_pct<span class="token punctuation">)</span> <span class="token string">&quot;annual_sal&quot;</span>
<span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><blockquote><p>这里你一定要注意，在 MySQL 里面， <strong>空值不等于空字符串</strong>。<strong>一个空字符串的长度是 0，而一个空值的长度是空</strong>。而且，<strong>在 MySQL 里面，空值是占用空间的</strong>。</p></blockquote> <h3 id="_3-5-着重号"><a href="#_3-5-着重号" class="header-anchor">#</a> 3.5 着重号</h3> <ul><li><p>错误的</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code>mysql<span class="token operator">&gt;</span> <span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> <span class="token keyword">ORDER</span><span class="token punctuation">;</span>
ERROR <span class="token number">1064</span> <span class="token punctuation">(</span><span class="token number">42000</span><span class="token punctuation">)</span>: You have an error <span class="token operator">in</span> your <span class="token keyword">SQL</span> syntax<span class="token punctuation">;</span> <span class="token keyword">check</span> the manual that
corresponds <span class="token keyword">to</span> your MySQL server version <span class="token keyword">for</span> the <span class="token keyword">right</span> syntax <span class="token keyword">to</span> <span class="token keyword">use</span> near <span class="token string">'ORDER'</span> at
line <span class="token number">1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div></li> <li><p>正确的</p> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049328.png" alt="image-20220124020537726"></p></li> <li><p>结论</p></li></ul> <blockquote><p><strong>我们需要保证表中的字段、表名等没有和保留字、数据库系统或常用方法冲突。如果真的相同，请在SQL语句中使用一对``（着重号）引起来。</strong></p></blockquote> <h3 id="_3-6-查询常数"><a href="#_3-6-查询常数" class="header-anchor">#</a> 3.6 查询常数</h3> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049329.png" alt="image-20220124020733838"></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token string">'atguigu'</span> <span class="token keyword">as</span> corporation<span class="token punctuation">,</span> last_name <span class="token keyword">FROM</span> employees<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h2 id="_4-显示表结构"><a href="#_4-显示表结构" class="header-anchor">#</a> 4. 显示表结构</h2> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049330.png" alt="image-20220124020844792"></p> <p>其中，各个字段的含义分别解释如下：</p> <blockquote><ul><li>Field：表示字段名称。</li> <li>Type：表示字段类型，这里 barcode、goodsname 是文本型的，price 是整数类型的。</li> <li>Null：表示该列是否可以存储NULL值。</li> <li>Key：表示该列是否已编制索引。PRI表示该列是表主键的一部分；UNI表示该列是UNIQUE索引的一</li> <li>部分；MUL表示在列中某个给定值允许出现多次。</li> <li>Default：表示该列是否有默认值，如果有，那么值是多少。</li> <li>Extra：表示可以获取的与给定列有关的附加信息，例如AUTO_INCREMENT等。</li></ul></blockquote> <h2 id="_5-过滤数据"><a href="#_5-过滤数据" class="header-anchor">#</a> 5. 过滤数据</h2> <ul><li><p>背景：</p> <p><img src="https://lcfimgsbed.oss-cn-chengdu.aliyuncs.com/img/202207242049331.png" alt="image-20220124020959289"></p></li> <li><p>语法：</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> 字段<span class="token number">1</span><span class="token punctuation">,</span>字段<span class="token number">2</span>
<span class="token keyword">FROM</span> 表名
<span class="token keyword">WHERE</span> 过滤条件
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><ul><li>使用WHERE 子句，将不满足条件的行过滤掉</li> <li><strong>WHERE子句紧随 FROM子句</strong></li></ul></li> <li><p>举例</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> employee_id<span class="token punctuation">,</span> last_name<span class="token punctuation">,</span> job_id<span class="token punctuation">,</span> department_id
<span class="token keyword">FROM</span>  employees
<span class="token keyword">WHERE</span> department_id <span class="token operator">=</span> <span class="token number">90</span> <span class="token punctuation">;</span>
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